To reduce discharge of fuel vapors into the atmosphere, motor vehicles induct fuel vapors from a fuel tank into the engine. An evaporative emission control system including a carbon canister is also coupled to the fuel tank to adsorb fuel vapors under some conditions when the internal combustion engine is not running. The carbon canister, however, has limited capacity, thus engine running manifold vacuum may be used to desorb the vapor from the carbon canister via opening of a purge valve. Desorbed vapors are combusted in engine.
Diagnostics may be performed on the evaporative emission control system, e.g., to detect leaks in the system. Leak diagnostics may be based on pressure or vacuum changes in one or more components of the emissions control system during certain conditions. The inventors herein have recognized that a common leak path in an emission control system is through a canister purge valve located in a conduit between a fuel vapor canister and the engine.
In some approaches, pressure readings from a pressure sensor in a fuel tank may be monitored during engine operation while the canister purge valve is commanded closed in order to determine if a leak is present in the canister purge valve. For example, if a leak is present in the purge valve while the purge valve is closed and the fuel tank is sealed off from the atmosphere, then a vacuum may build in the fuel tank during engine operation which is indicative of a leak in the purge valve. The inventors herein have recognized that such approaches rely on a pressure sensor in the fuel tank to diagnose leaks and if the pressure sensor degrades then leak testing may be compromised. Thus, it may be desirable to provide an alternative approach to detecting leaks in a purge valve which does not rely on pressure sensors in the fuel tank.
In one example approach to at least partially address these issues, a method for a vehicle with an engine comprises indicating a leak in response to a temperature change in a fuel vapor canister coupled to a fuel tank in an emission control system while the engine is in operation and a purge valve is closed. For example, a leak in the purge valve may be indicated in response to a temperature decrease in the fuel vapor canister while the engine is in operation and the purge valve is closed. In this way, the technical result of a leak in a canister purge valve being indicated even when a fault is present in a pressure sensor in the fuel tank can be obtained by using a temperature sensor, e.g., a thermocouple, in the canister to monitor temperature changes in the canister when the purge valve is closed.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.